1. Technical Field
The present invention relates to a wireless communication system, wireless communication device, and a wireless communication method.
2. Background Art
Recently, many wired communication networks have been replaced by wireless communication networks. In particular, with the release of apparatuses that have only a wireless communications interface, such as tablet devices and smartphones, it is projected that wireless communication networks will continue to be increasingly used. The transfer rate of wireless communication networks has been enhanced spectacularly in accordance with increasing demand. As a result, it has become possible to transfer large-sized data such as images and video and real-time data such as audio using wireless communication.
Wireless communication circuits in terminal devices included in wireless communication systems (wireless transmitters and wireless receivers) perform communication in synchronization with a predetermined series of time slots. Furthermore, in case of transferring data including both video and audio signals in synchronized time slots wirelessly, it is necessary to synchronize an encoder in the wireless transfer device with a decoder in the wireless receiver in order to synchronize the video signal with the audio signal. MPEG-2 Transport Stream (TS) technology is widely used as a technique that synchronizes the video signal with the audio signal and used in fields such as digital broadcasting. TS packets that include the video signal and the audio signal compressed using MPEG-2 and H.264 technology are transferred via a network.
In addition to the video signal and the audio signal, various other data is stored in the TS packets, including a Program Clock Reference (PCR) value. The PCR value indicates current time information of the encoder. The decoder controls output timing of the video signal and audio signal by adjusting an internal timer value using the PCR value and synchronizes the video signal with the audio signal.
By adopting MPEG-2-TS technology in the wireless communication system, it is possible to perform encoding and decoding of data in real time. However, in order to implement high-quality encoding and decoding in real time, it is necessary to minimize packet dropout on the wireless communication network and absorb fluctuation of propagation time in wireless transmission.
The video signal is compressed along the time axis to shrink the bandwidth necessary for wireless transmission. However, if TS packets are dropped due to frame dropout, an error occurs in decoding the video signal. In addition, if the reference image data is dropped, the follow-on video data cannot be decoded correctly.
In some cases, due to the fluctuation of the propagation time in wireless transmission, quality of the video signal and audio signal cannot be maintained. In MPEG-2-TS technology, as described above, the decoder absorbs skew in the clock signal to follow the time information of the encoder using the PCR value, and the decoder is synchronized with the encoder. However, if the propagation time of the TS packet that stores the PCR value varies, its arrival time also varies, and the time information of the encoder acquired from the PCR value varies as well. In this case, since the decoder follows the varied time, oscillating frequency of the clock signal goes up and down substantially, and that results in distorting the video signal and causing color shift. To cope with this issue, in the MPEG-2-TS specification, it is stipulated that PCR tolerance is to be kept to within several tens of μsec.